Observation of a moderate Major Baltic Sea inflow in December 2023

Abstract Just before Christmas 2023, the low-pressure system storm "Zoltan" struck Germany, resulting in widespread damage and two consecutive large storm surges on the North Sea coast in the night from Thursday 21 December 2023 to Friday 22 December 2023. Storm Zoltan brought heavy rainfall, accompanied by thunderstorms and winds ranging between 90 and 115 km h-1, with gusts reaching up to 140 km h-1 along the coast. Besides major rail service disruption and Christmas market clousers, it caused extreme flooding in northern Germany, e.g., in Hamburg St. Pauli sea level height rose to more than 3 m above mean high water level on 22 December 2023, flooding the historic Hamburg fish market. Characteristics of the inflowing water at the Darss Sill (DAR) and the Arkona basin buoy (ARK), including salinity, temperature, oxygen concentration and current properties, were analysed to understand the impact of storm Zoltan in the western Baltic Sea.In addition to the damage along its path, following the onset of strong westerly winds associated with storm Zoltan, a large volume of water, containing saline (17-22 psu), cold (5-6°C), and oxygen-rich (7-8 ml l-1) water from the North Sea reaches into the western Baltic Sea. The sea level at Landsort Norra increased by +57 cm over a period of 14 days, from December 15 to December 29 2023. This results in a total volume change of 198 $\mathrm{km^3}$ in the Baltic Sea, with 169 $\mathrm{km^3}$ and 29 $\mathrm{km^3}$ provided via the Belt Sea and the Sound Sea, respectively. Observations at the Darss Sill station indicate a significant inflow between 19 December 2023 and 1 January 2024 with salinity above 13 psu, temperature below 5.5°C and dissolved oxygen concentration of about 7.5-8 ml l-1. While the maximum salinity of the bottom layer at Darss Sill is about 17 psu, the Arkona Basin buoy exhibits significantly higher salinities, reaching up to 22 psu at the bottom layer. During the main inflow period, 75 km3 of highly saline water enters the western Baltic Sea. This corresponds to a salt transport of 1.65 Gt into the western Baltic Sea (1.33 Gt from the Belt Sea and 0.32 Gt from the Sound Sea), which classifies the event in the moderate range of Major Baltic Sea inflows (MBI). This is the strongest inflow into the Baltic Sea since 2016.